The present invention is concerned with pistons for engines or motors, for example, internal combustion engines, or hydraulic or pneumatic reciprocating motors. In particular, the invention is concerned with pistons comprising a crown and a skirt. Such pistons are normally made with the crown and skirt integral and made of metal, e.g. an alloy of aluminium. It is known, however, for the skirt to be of a different material from the crown, e.g. GB 2129523A describes a piston with a metal crown and a plastics skirt. It is also known to provide a skirt with pads which extend from a thrust surface of the skirt (see GB 2104188A). The pads have the purpose of transmitting lateral thrusts of the piston against the wall of an associated cylinder. GB 2162614A describes providing each pad, there are typically three on each thrust surface of the skirt, as a plastics insert in the skirt. In addition., in order to reduce wear of the plastic inserts it is disclosed that each plastic insert is bevelled apropriately to cause provided lubricant to be forced over the plastic insert by a hydrodynamic action during the reciprocation of the piston. The inserts have to be individually mounted on the skirt, each in an individual recess which is an undesirably complex operation.
The presence of the plastics inserts, due to compliancy or conformability, can allow smaller operating clearances in the cylinder bore than otherwise would be the case. However, it is required that the plastics inserts are sufficiently stiff to withstand thrust loading without significant distortion occurring which, in turn, depends on the thickness and composition of the inserts. The provision of smaller operating clearances implies that piston slap noise is reduced, and any slap impact is also quietened by the natural damping characteristics of the plastics material. Further, improved piston stability, because of the provision of smaller operating clearances, can enhance the seating capabilities of the ring pack provided with the piston.
In any such known arrangement, the total area of the inserts is only a small proportion of the area of the piston skirt.
Lighter pistons are advantageous in that the reciprocating mass of the engine or motor is reduced, improving fuel consumption, noise, and increasing the possible speed of operation. Further, it is possible to employ lighter connecting rods and smaller bearings, in association with lighter pistons.
It is an object of the present invention to provide a piston having pads of a different material from the remainder of the skirt which is of simple construction.